Recombination and microstructural properties of soldering pads and their impact on solar cell performance
Abstract
The recombination induced by soldering pads, screen printed with a commercially available paste containing primarily silver and an aluminium amount of 1-5%wt, is investigated concerning the open-circuit voltage for aluminium back surface field (Al-BSF) solar cells. The saturation current density Jo,pad due to soldering pads is found to be in the range of 26.7 103 to 85.4 103 fA/cm2 for different conditions of the rear silicon surface. The recombination under soldering pads leads to a voltage drop of DVoc = 6.7 mV for Al-BSF solar cells with 6.2% of the rear side being covered with soldering pads compared to a cell with full-area Al-BSF. Within a microstructural analysis of the interface between the silicon surface and the soldering pads it is found that Al-alloying from the Al share in the soldering pad paste only occurs insularly. Furthermore, Al-BSF cells with and without residual rear side emitter and varying area fraction of soldering pads, using a soldering pad paste with and without an Al-share, are investigated. It is found that the non-overcompensated emitter regions under the soldering pads of Al-BSF solar cells affect cell performance dramatically in case of full area soldering pad, especially for an Al-free pad paste.
